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X-shaped benzoylbenzophenone derivatives with crossed donors and acceptors for highly efficient thermally activated delayed fluorescence Sae Youn Lee,a Takuma Yasuda,a,b,c In Seob Park a and Chihaya Adachia,b
DOI: 10.1039/x0xx00000x www.rsc.org/
Thermally activated delayed fluorescence (TADF) materials based on benzoylbenzophenone, AcPmBPX and PxPmBPX, were designed and synthesized. Organic light-emitting diodes using these materials as emitters exhibited high external electroluminescence quantum efficiencies of up to 11%. Since the first seminal report on organic light-emitting diodes (OLEDs) using tris(8-quinolinolato)aluminum(III) as an emitter,1 a vast variety of organic luminescent molecules have been developed and tested for application to OLEDs. For many years, the OLEDs with the highest external electroluminescence (EL) quantum efficiencies (ηext) utilized phosphorescent materials like organometallic complexes incorporating heavy metals such as iridium(III) and platinum(II).2–4 In phosphorescent OLEDs, singlet (S1) and triplet (T1) excitons formed, according to spin statistics, in a ratio of 1:3 by the recombination of holes and electrons under electrical excitation can be harvested by fast intersystem crossing (ISC) through the strong spin-orbital coupling of the heavy-metal center.5,6 Therefore, phosphorescent materials can harvest nearly 100% of excitons, while fluorescent materials use only 25% of the a
Department of Applied Chemistry and Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395 Japan E-mail: [email protected], [email protected] b International Institute for Carbon Neutral Energy Research (WPII2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan c INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan † Electronic Supplementary Information (ESI) available. For synthesis and characterization details of materials see DOI: 10.1039/c000000x/
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excitons, i.e., the singlets, for light emission.7 However, phosphorescent OLEDs suffer from inferior stability and efficiency in the blue emission region because of effects such as strong exction-charge carrier interaction (triplet–triplet exciton annihilation (TTA) and triplet–polaron annihilation (TPA)) and exciton dissociation by the electric field that lead to excessive roll-off at high current densities.8−11 Recently, we reported OLEDs exploiting the often disregarded luminescence mechanism of thermally activated delayed fluorescence (TADF),12 which can also harvest both S1 and T1 excitons for fluorescence emission by thermally up-converting T1 states to emissive S1 states through reverse ISC (RISC). We have since designed a number of pure organic luminophores, including azaheteroaromatic,13 sulfon,14 and cyanobenzene15 derivatives, exhibiting efficient TADF. For efficient RISC, a small energy gap (∆EST) between the lowest excited S1 and T1 states is needed. Small ∆EST can be attained by minimizing the exchange energy between the S1 and T1 states, which can be achieved by reducing the special overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the case of benzophenone, the electronic transition from a non-bonding n orbital to a π* orbital (i.e., n-π* transition) possesses a small ∆EST as a consequence of the orthogonal overlap between n and π* orbitals.16 This small ∆EST of benzophenone can induce delayed fluorescence emission, but the emission is very weak because of strong thermal deactivation processes at room temperature.17,18 However, by employing a donor-acceptor-donor (D-A-D) quadrupolar electronic structure, we obtained efficient TADF emission and OLED performance covering the full color range from benzophenone derivatives used as emitters in TADF-OLEDs.19
J. Name., 2015, 00, 1-3 | 1
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In this study, we focus on the advanced molecular design of benzoyl-benzophenone-based TADF luminophores with donors and acceptors crossed in an X-shaped molecular structure. The new molecules 1,4-Bis(9,9-dimethylacridan-10-yl-p-phenyl)2,5-bis(p-tolyl-methanoyl)benzene (AcPmBPX, 1) and 1,4Bis(9,9-phenoxazin-10-yl-p-phenyl)-2,5-bis(p-tolylmethanoyl)benzene (PxPmBPX, 2), shown in Fig. 1a, exhibit sky-blue and green emission in toluene solution (Fig. 1b). The geometric and electronic structures of 1 and 2 were ascertained by time-dependent density functional theory (TDDFT) calculations performed at the B3LYP/6-31G(d,p) level. As shown in Fig. 1a, the HOMOs of 1 and 2 are mainly distributed over the electron-donating dimethylacridane and phenoxazine moieties, respectively, because of a large distortion between the electron-donating moieties and neighboring phenylene rings. The LUMOs of 1 and 2 are located on the central electron-accepting benzoylbenzophenone core. Such clear spatial separation of the frontier orbitals resulted in a very small calculated ∆EST of 0.01 eV for 1 and 2, which can facilitate fast RISC from T1 to S1 states even at room-temperature.
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The UV/Vis absorption and Photoluminescence (PL) spectra of 1 and 2 in toluene are depicted in Fig. 1b. Compounds 1 and 2 exhibit structure-less sky-blue and green emission with maxima (λPL) at 496 and 510 nm, respectively. In oxygen-free toluene solutions, PL quantum efficiencies (ΦPL) of ~20% were obtained for 1 and 2, whilst ΦPL decreased to less than 10% in the presence of triplet oxygen (3O2) in aerated solutions. In contrast, films of 1 and 2 doped into mCBP as host (mCBP: 3,3’-bis(carbazol-9-yl)biphenyl) exhibited high ΦPL of 46% and 57%, respectively, which suggests that non-radiative exciton quenching processes are suppressed by the restriction of intramolecular rotation in the solid state.20 We further analyzed the temperature dependence of the transient PL characteristics of 1 and 2 doped into films of mCBP using a streak camera. As shown in Fig. 2, a 6 wt% 2:mCBP co-deposited film exhibits fluorescence with nanosecond-scale prompt (lifetime of τ p = 30 ns) and microsecond-scale delayed (τd = 314 µs) components at 300 K. Since the prompt and delayed emission spectra coincide with each other (Fig. 2a), the delayed PL component can be ascribed to TADF emission. Moreover, ΦPL increased with increasing temperature in the range of 150–300 K because of an acceleration of the RISC process at higher temperatures (Fig. 2b). These results indicate that the delayed component
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Fig. 3 (a) Energy level diagram of OLEDs and chemical structures of the materials used for the devices. (b) Current-density–voltage– luminance (J–V–L) curves of OLEDs based on the TADF emitters 1 (black) and 2 (red). (c) External EL quantum efficiency (ηext) vs. current-density plots (inset: normalized EL spectra measured at 10 mA cm-2).
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originates from TADF. Using the ΦPL and lifetimes of the 6 wt% 2:mCBP co-deposited film at 300 K, the rate constants of radiative decay from the S1 state (krS), ISC (kISC), and RISC (kRISC) are estimated to be 7.0 × 106, 2.6 × 107, and 7.0 × 103 s‒1, respectively (ESI†). Because of their high ΦPL in doped films, emitters 1 and 2 were expected to be useful for fabricating highly efficient TADF-OLEDs. To test their performance as emitters in OLEDs, we fabricated multilayer OLEDs with the following device configuration (Fig. 3a): indium-tin-oxide (ITO)/ 4,4’-bis[N-(1naphthyl)-N-phenyl]biphenyl diamine (α-NPD, 35 nm)/ 1,3bis(carbazol-9-yl)benzene (mCP, 10 nm)/ 6 wt% 1 or 2:mCBP (20 nm)/ PPF (10 nm)/ 1,3,5-tris(N-phenylbenzimidazol-2yl)benzene (TPBi, 30 nm)/ LiF (0.8 nm)/ Al (80 nm). Here, αNPD and TPBi serve as a hole- and electon-transporting layers (HTL and ETL), respectively, and LiF is used as an electroninjecting material. In addition, to confine T1 excitons to the emitting layer (EML) and to suppress quenching by the neighboring layers, mCP and PPF, which have high T1 energies of 2.9 and 3.1 eV,21 respectively, were inserted between the HLT/EML and EML/ETL interfaces, respectively. Figures 3b and c present the current-density–voltage– luminance (J–V–L) and external quantum efficiency (ηext) characteristics of the OLEDs based on TADF emitters 1 and 2. For the 1- and 2-based devices, the EL maxima were observed at 504 and 541 nm at J = 10 mA cm−2, which are similar to the corresponding λPL. The OLEDs based on 1 and 2 achieved high maximum ηext of 10.0% and 11.3%, respectively. The 2-based device also exhibited a high current efficiency (ηc) of 35.3 cd A‒1 and a high maximum luminance (Lmax) of 61,040 cd m‒2 (ESI†). In the case of the 1-based device, ηext decreased at high current densities over 100 mA cm‒2 (ηext < 3%) and was wellfitted by an exciton quenching model based on combination of singlet−triplet annihilation (STA) and TTA (Fig. S3),22 indicating that the longer luminescence lifetime in doped films for emitter 1 (τd = 925 µs) compared to 2 (τd = 314 µs, ESI†) accelerates strong exciton-charge carrier interactions and electric-field-assisted dissociation, which leads to a rapid increase of roll-off.10,11 Even though strong roll-off was observed at high current densities in the 1-based device, its maximum ηext was 10.0%, which is still more than double the 5% theoretical limit of ηext for OLEDs based on conventional fluorescent emitter. In summary, the X-shaped molecules of 1 and 2 with crossed donors and acceptors were designed and synthesized. The large steric hindrance of the bulky benzoylbenzophenone core units leads to strong distortion between acceptor and donor units, thereby realizing relatively small experimentally-determined ∆EST of 0.05 and 0.02 eV, which promote efficient TADF emission. The OLEDs based on 1 and 2 as emitters exhibited high ηext of up to 11% for green and yellow emission. We believe that this molecular design strategy offers a way to develop light-emitting materials with small ∆EST for highperformance TADF-OLEDs.
J. Name., 2012, 00, 1-3 | 3
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Acknowledgements
Dalton Transactions Accepted Manuscript
This work was supported by a Grant-in-Aid from the Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST) and by grants from JSPS Young Scientists (A) (no. 25708032) and Challenging Exploratory Research (no, 26620168). C.A. and T.Y. acknowledge the support of WPII2CNER, sponsored by MEXT (Japan).
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Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913. M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Silbley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151. M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4. J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau and M. E. Tompshon, J. Am. Chem. Soc, 2001, 123, 4304. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys., 2001, 90, 5048. L. Xiao, S.-J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009, 21, 1271. C. Adachi, M. A. Baldo, S. R. Forrest, M. E. Thompson, Appl. Phys. Lett., 2000, 77, 904. S. Reineke, K. Walzer and K. Leo, Phys. Rev. B, 2007, 75, 125328. C. Murawski, K. Leo and M. C. Gather, Adv. Mater., 2013, 25, 6801. J. Kalinowski, W. Stampor, and J. Mezyk, Phys. Rev. B, 2002, 66, 235321. J. Kalinowski, W. Stampor, and J. Szmytkowski, Phys. Rev. B, 2006, 74, 085316. A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato and C. Adachi, Adv. Mater., 2009, 21, 4802. S. Y. Lee, T. Yasuda, H. Nomura and C. Adachi, Appl. Phys. Lett., 2012, 101, 093306. Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka and C. Adachi, Nat. Photon., 2014, 8, 326. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature. 2012, 492, 234. N. J. Turro, Modern molecular photochemistry, 1991, University science book. pp. 29. J. Saltiel, H. C. Curtis, L. Metts, J. W. Miley, J. Winterle and M. Wrighton, J. Am. Chem. Soc., 1970, 92, 410. M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer and J. H. Parks, J. Am. Chem. Soc., 1975, 97, 4490. S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang, C. Adachi, Angew. Chem. Int. Ed. 2014, 126, 6520. Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009, 4332. P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak and P. E. Burrows, Org. Lett., 2006, 8, 4211. K. Masui, H. Nakanotani and C. Adachi, Org. Electron., 2013, 14, 2721.
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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 2012, Accepted 00th January 2012
X-shaped benzoylbenzophenone derivatives with crossed donors and acceptors for highly efficient thermally activated delayed fluorescence Sae Youn Lee,a Takuma Yasuda,a,b,c In Seob Park a and Chihaya Adachia,b
DOI: 10.1039/x0xx00000x www.rsc.org/
Thermally activated delayed fluorescence (TADF) materials based on benzoylbenzophenone, AcPmBPX and PxPmBPX, were designed and synthesized. Organic light-emitting diodes using these materials as emitters exhibited high external electroluminescence quantum efficiencies of up to 11%. Since the first seminal report on organic light-emitting diodes (OLEDs) using tris(8-quinolinolato)aluminum(III) as an emitter,1 a vast variety of organic luminescent molecules have been developed and tested for application to OLEDs. For many years, the OLEDs with the highest external electroluminescence (EL) quantum efficiencies (ηext) utilized phosphorescent materials like organometallic complexes incorporating heavy metals such as iridium(III) and platinum(II).2–4 In phosphorescent OLEDs, singlet (S1) and triplet (T1) excitons formed, according to spin statistics, in a ratio of 1:3 by the recombination of holes and electrons under electrical excitation can be harvested by fast intersystem crossing (ISC) through the strong spin-orbital coupling of the heavy-metal center.5,6 Therefore, phosphorescent materials can harvest nearly 100% of excitons, while fluorescent materials use only 25% of the a
Department of Applied Chemistry and Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395 Japan E-mail: [email protected], [email protected] b International Institute for Carbon Neutral Energy Research (WPII2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan c INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan † Electronic Supplementary Information (ESI) available. For synthesis and characterization details of materials see DOI: 10.1039/c000000x/
This journal is © The Royal Society of Chemistry 2015
excitons, i.e., the singlets, for light emission.7 However, phosphorescent OLEDs suffer from inferior stability and efficiency in the blue emission region because of effects such as strong exction-charge carrier interaction (triplet–triplet exciton annihilation (TTA) and triplet–polaron annihilation (TPA)) and exciton dissociation by the electric field that lead to excessive roll-off at high current densities.8−11 Recently, we reported OLEDs exploiting the often disregarded luminescence mechanism of thermally activated delayed fluorescence (TADF),12 which can also harvest both S1 and T1 excitons for fluorescence emission by thermally up-converting T1 states to emissive S1 states through reverse ISC (RISC). We have since designed a number of pure organic luminophores, including azaheteroaromatic,13 sulfon,14 and cyanobenzene15 derivatives, exhibiting efficient TADF. For efficient RISC, a small energy gap (∆EST) between the lowest excited S1 and T1 states is needed. Small ∆EST can be attained by minimizing the exchange energy between the S1 and T1 states, which can be achieved by reducing the special overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the case of benzophenone, the electronic transition from a non-bonding n orbital to a π* orbital (i.e., n-π* transition) possesses a small ∆EST as a consequence of the orthogonal overlap between n and π* orbitals.16 This small ∆EST of benzophenone can induce delayed fluorescence emission, but the emission is very weak because of strong thermal deactivation processes at room temperature.17,18 However, by employing a donor-acceptor-donor (D-A-D) quadrupolar electronic structure, we obtained efficient TADF emission and OLED performance covering the full color range from benzophenone derivatives used as emitters in TADF-OLEDs.19
J. Name., 2015, 00, 1-3 | 1
Dalton Transactions Accepted Manuscript
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Transactions DOI:Dalton 10.1039/C4DT03608E
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In this study, we focus on the advanced molecular design of benzoyl-benzophenone-based TADF luminophores with donors and acceptors crossed in an X-shaped molecular structure. The new molecules 1,4-Bis(9,9-dimethylacridan-10-yl-p-phenyl)2,5-bis(p-tolyl-methanoyl)benzene (AcPmBPX, 1) and 1,4Bis(9,9-phenoxazin-10-yl-p-phenyl)-2,5-bis(p-tolylmethanoyl)benzene (PxPmBPX, 2), shown in Fig. 1a, exhibit sky-blue and green emission in toluene solution (Fig. 1b). The geometric and electronic structures of 1 and 2 were ascertained by time-dependent density functional theory (TDDFT) calculations performed at the B3LYP/6-31G(d,p) level. As shown in Fig. 1a, the HOMOs of 1 and 2 are mainly distributed over the electron-donating dimethylacridane and phenoxazine moieties, respectively, because of a large distortion between the electron-donating moieties and neighboring phenylene rings. The LUMOs of 1 and 2 are located on the central electron-accepting benzoylbenzophenone core. Such clear spatial separation of the frontier orbitals resulted in a very small calculated ∆EST of 0.01 eV for 1 and 2, which can facilitate fast RISC from T1 to S1 states even at room-temperature.
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The UV/Vis absorption and Photoluminescence (PL) spectra of 1 and 2 in toluene are depicted in Fig. 1b. Compounds 1 and 2 exhibit structure-less sky-blue and green emission with maxima (λPL) at 496 and 510 nm, respectively. In oxygen-free toluene solutions, PL quantum efficiencies (ΦPL) of ~20% were obtained for 1 and 2, whilst ΦPL decreased to less than 10% in the presence of triplet oxygen (3O2) in aerated solutions. In contrast, films of 1 and 2 doped into mCBP as host (mCBP: 3,3’-bis(carbazol-9-yl)biphenyl) exhibited high ΦPL of 46% and 57%, respectively, which suggests that non-radiative exciton quenching processes are suppressed by the restriction of intramolecular rotation in the solid state.20 We further analyzed the temperature dependence of the transient PL characteristics of 1 and 2 doped into films of mCBP using a streak camera. As shown in Fig. 2, a 6 wt% 2:mCBP co-deposited film exhibits fluorescence with nanosecond-scale prompt (lifetime of τ p = 30 ns) and microsecond-scale delayed (τd = 314 µs) components at 300 K. Since the prompt and delayed emission spectra coincide with each other (Fig. 2a), the delayed PL component can be ascribed to TADF emission. Moreover, ΦPL increased with increasing temperature in the range of 150–300 K because of an acceleration of the RISC process at higher temperatures (Fig. 2b). These results indicate that the delayed component
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Fig. 3 (a) Energy level diagram of OLEDs and chemical structures of the materials used for the devices. (b) Current-density–voltage– luminance (J–V–L) curves of OLEDs based on the TADF emitters 1 (black) and 2 (red). (c) External EL quantum efficiency (ηext) vs. current-density plots (inset: normalized EL spectra measured at 10 mA cm-2).
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originates from TADF. Using the ΦPL and lifetimes of the 6 wt% 2:mCBP co-deposited film at 300 K, the rate constants of radiative decay from the S1 state (krS), ISC (kISC), and RISC (kRISC) are estimated to be 7.0 × 106, 2.6 × 107, and 7.0 × 103 s‒1, respectively (ESI†). Because of their high ΦPL in doped films, emitters 1 and 2 were expected to be useful for fabricating highly efficient TADF-OLEDs. To test their performance as emitters in OLEDs, we fabricated multilayer OLEDs with the following device configuration (Fig. 3a): indium-tin-oxide (ITO)/ 4,4’-bis[N-(1naphthyl)-N-phenyl]biphenyl diamine (α-NPD, 35 nm)/ 1,3bis(carbazol-9-yl)benzene (mCP, 10 nm)/ 6 wt% 1 or 2:mCBP (20 nm)/ PPF (10 nm)/ 1,3,5-tris(N-phenylbenzimidazol-2yl)benzene (TPBi, 30 nm)/ LiF (0.8 nm)/ Al (80 nm). Here, αNPD and TPBi serve as a hole- and electon-transporting layers (HTL and ETL), respectively, and LiF is used as an electroninjecting material. In addition, to confine T1 excitons to the emitting layer (EML) and to suppress quenching by the neighboring layers, mCP and PPF, which have high T1 energies of 2.9 and 3.1 eV,21 respectively, were inserted between the HLT/EML and EML/ETL interfaces, respectively. Figures 3b and c present the current-density–voltage– luminance (J–V–L) and external quantum efficiency (ηext) characteristics of the OLEDs based on TADF emitters 1 and 2. For the 1- and 2-based devices, the EL maxima were observed at 504 and 541 nm at J = 10 mA cm−2, which are similar to the corresponding λPL. The OLEDs based on 1 and 2 achieved high maximum ηext of 10.0% and 11.3%, respectively. The 2-based device also exhibited a high current efficiency (ηc) of 35.3 cd A‒1 and a high maximum luminance (Lmax) of 61,040 cd m‒2 (ESI†). In the case of the 1-based device, ηext decreased at high current densities over 100 mA cm‒2 (ηext < 3%) and was wellfitted by an exciton quenching model based on combination of singlet−triplet annihilation (STA) and TTA (Fig. S3),22 indicating that the longer luminescence lifetime in doped films for emitter 1 (τd = 925 µs) compared to 2 (τd = 314 µs, ESI†) accelerates strong exciton-charge carrier interactions and electric-field-assisted dissociation, which leads to a rapid increase of roll-off.10,11 Even though strong roll-off was observed at high current densities in the 1-based device, its maximum ηext was 10.0%, which is still more than double the 5% theoretical limit of ηext for OLEDs based on conventional fluorescent emitter. In summary, the X-shaped molecules of 1 and 2 with crossed donors and acceptors were designed and synthesized. The large steric hindrance of the bulky benzoylbenzophenone core units leads to strong distortion between acceptor and donor units, thereby realizing relatively small experimentally-determined ∆EST of 0.05 and 0.02 eV, which promote efficient TADF emission. The OLEDs based on 1 and 2 as emitters exhibited high ηext of up to 11% for green and yellow emission. We believe that this molecular design strategy offers a way to develop light-emitting materials with small ∆EST for highperformance TADF-OLEDs.
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Transactions DOI:Dalton 10.1039/C4DT03608E
Acknowledgements
Dalton Transactions Accepted Manuscript
This work was supported by a Grant-in-Aid from the Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST) and by grants from JSPS Young Scientists (A) (no. 25708032) and Challenging Exploratory Research (no, 26620168). C.A. and T.Y. acknowledge the support of WPII2CNER, sponsored by MEXT (Japan).
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